1. Field of the Invention
This invention relates to stabilization of thermal images. More particularly, this invention relates to thermal imaging media, processes for forming images and imaged media in which metal cations are used to reduce fading of the images during projection of the image by passage of visible radiation through the image. The sensitivity of some of the thermal imaging media of the invention is improved by the incorporation of the metal cations therein.
2. References to Related Applications
Copending patent application U.S. Ser. Nos. 07/695,641; 07/696,196 and 07/695,932 (now U.S. Pat. No. 5,153,169), all filed May 6, 1991 and all assigned to the same assignee as the present application, describe and claim imaging media comprising a color-forming layer comprising a thermal color-forming composition adapted to undergo a change of color upon increase in the temperature of the color-forming layer above a color-forming temperature for a color-forming time. Preferred imaging media described in these three applications are substantially as shown in FIG. 1 of the accompanying drawings and comprise three separate color-forming layers containing yellow, cyan and magenta thermal color-forming compositions; each of these color-forming compositions comprises a color-forming compound which can produce the desired color and an infra-red absorber capable of absorbing infra-red radiation and thereby generating heat in the color-forming layer. The three color-forming layers use infra-red absorbers absorbing at differing wavelengths so that each color-forming layer can be imaged independently; for example, specific imaging media disclosed in these three applications use infra-red absorbers having peak absorptions at approximately 792, 822 and 869 nm.
Copending application U.S. Ser. No. 07/696,222, filed May 6, 1991, and assigned to the same assignee as the present application, describes and claims certain processes for the synthesis of bis(heterocyclic) dyes, especially asymmetric dyes in which the two heterocyclic nuclei differ. These processes are useful for the synthesis of certain infra-red dyes used in the imaging medium of the present invention shown in FIG. 1 of the accompanying drawings.
Copending application U.S. Ser. No. 07/795,038, of even date herewith and assigned to the same assignee as the present application, describes and claims certain bis(benzpyrylium) infra-red dyes, including the croconate dye used in the thermal imaging medium described below with reference to FIG. 1 of the accompanying drawings.
Copending application U.S. Ser. No. 07/795,034, of even date herewith and assigned to the same assignee as the present application, describes and claims certain amino-substituted squarylium infra-red dyes, including the dyes of Formulae IR2, IR3 and IR5 used in the thermal imaging medium described below with reference to FIG. 1 of the accompanying drawings.
Copending application U.S. Ser. No. 07/696,151, filed May 6, 1991, and assigned to the same assignee as the present application, describes and claims leuco dyes which can be used in the imaging medium of the present invention.
Copending application U.S. Ser. No. 07/277,014 (now abandoned), filed Nov. 28, 1988, and assigned to the same assignee as the present application, describes and claims the yellow leuco dye used in the imaging medium of the present invention shown in FIG. 1 of the accompanying drawings.
U.S. Pat. No. 5,063,090, assigned to the same assignee as the present application, describes and claims quinophthalone leuco dyes which can be used in the imaging medium of the present invention.
Copending application U.S. Ser. No. 07/795,102 of even date herewith and assigned to the same assignee as the present application describes and claims thermal imaging media generally similar to those of the present invention, but in which quinones and hydroquinones, rather than certain metal cations, are used to increase the sensitivity of, and reduce fading of images formed from, the imaging media.
The disclosures of all the aforementioned copending applications are herein incorporated by reference.
3. Description of the Prior Art
As already indicated, imaging media are known which have at least one color-forming layer comprising a color-forming composition adapted to undergo a change of color (from colorless to colored, from colored to colorless, or from one color to another) upon increase in the temperature of the color-forming layer above a color-forming temperature for a color-forming time. The color change in such media need not be supplied by applying heat directly to the medium; the color-forming composition may comprise a color-forming compound (also referred to herein as a "leuco dye") which undergoes a change of color upon heating above a color-forming temperature, and an absorber capable of absorbing actinic (usually infra-red) radiation and thereby generating heat in the color-forming layer. When such a medium is exposed to appropriate actinic radiation, this radiation is absorbed by the absorber, thereby heating the color-forming compound and causing it to undergo its color change. Many such thermal imaging media have the advantage over conventional silver halide media of not requiring a post-exposure developing step. Such thermal imaging media also have the advantage that they are essentially insensitive to visible light, so that they can be handled under normal lighting conditions.
For example U.S. Pat. Nos. 4,602,263 and 4,826,976 both describe thermal imaging systems for optical recording and particularly for forming color images. These thermal imaging systems rely upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift. U.S. Pat. Nos. 4,720,449 and 4,960,901 describe a similar imaging system in which the color-developing component is a substantially colorless di- or triarylmethane imaging compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom to form a 5- or 6-membered ring, said moiety possessing a nitrogen atom bonded directly to the meso carbon atom and the nitrogen atom being bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position that cannot bond to the meso carbon atom, whereby the di- or triarylmethane compound is rendered colored. Other thermal imaging systems using di- or triarylmethane compounds are described in U.S. Pat. No. 4,720,450, while U.S. Pat. No. 4,745,046 describes a thermal imaging system using as color-forming co-reactants a substantially colorless di- or triarylmethane compound possessing on the meso carbon atom within its di- or triarylmethane structure an aryl group substituted in the ortho position with a nucleophilic moiety which is ring-closed on the meso carbon atom, and an electrophilic reagent which, upon heating and contacting the di- or triarylmethane compound, undergoes a bimolecular nucleophilic substitution reaction with the nucleophilic moiety to form a colored, ring-opened di- or triarylmethane compound.
The aforementioned patents describe a preferred form of imaging medium for forming multicolor images; in this preferred imaging medium, three separate color-forming layers, capable of forming yellow, cyan and magenta dyes, respectively, are superposed on top of one another. Each of the three color-forming layers has an infra-red absorber associated therewith, these absorbers absorbing at differing wavelengths, for example 760, 820 and 880 nm. This medium is imagewise exposed to three lasers having wavelengths of 760, 820 and 880 nm. (In the present state of technology, solid state diode lasers emitting at about 760 to 1000 nm provide the highest output per unit cost. Since most of the color-forming materials described in the aforementioned patents do not have high extinction coefficients within this wavelength range, it is necessary to include the infra-red absorbers with the leuco dyes in order to ensure efficient absorption of the laser radiation and hence efficient heating of the leuco dye.) The resultant imagewise heating of the color-forming layers causes the leuco dyes to undergo color changes in the exposed areas, thereby producing a multicolored image, which needs no development.
This preferred type of imaging medium is capable of very high resolution images; for example, the medium can readily be used to produce a 2K line 35 mm slide (i.e., a slide having 2000 pixels in each line parallel to the long edges of the slide). However, it has been found that images produced from certain leuco dyes, especially those described in the aforementioned U.S. Pat. Nos. 4,720,449 and 4,960,901, tend to fade and/or undergo color shifts when those images are projected using powerful conventional slide projectors, for example xenon arc projectors, for extended periods of time. Obviously, fading and color shifts are undesirable and the need therefore exists for ways of preventing or at least reducing such fading and color shifts.
The thermal color-forming reactions described in the aforementioned patents do not provide any amplification such as occurs in silver halide based imaging media, and consequently the media are relatively insensitive; typically, the thermal media require energy inputs of about 1 J/cm.sup.2 per color-forming layer to achieve maximum transmission optical densities around 3.0, which are needed for acceptable slides. Accordingly, it would be advantageous to improve the sensitivity of these thermal imaging media so as to improve the speed of image formation and/or reduce the power requirements for the energy source used for imaging.
It has now been found that certain metal cations reduce the fading and color shifts which otherwise occur during projection of thermal images produced as described in the aforementioned U.S. Pat. Nos. 4,720,449 and 4,960,901, and also serve to increase the sensitivity of the thermal imaging media described in these patents.